The present invention relates to a power molding method and, more particularly, to method and apparatus for controlling a powder molding press for producing stepped mold products.
A conventional powder molding press of the above type is shown in FIG. 26 of the type such as disclosed in the Japanese Patent Laid-Open Publication No. 53-808767. Referring to FIG. 26, powder 101 filled in a die hole 110 of a die 100 disposed in a die plate is pressed by an upper punch assembly 102 and a lower punch assembly 103. The lower punch assembly 103 comprises cylindrical first, second and third lower punches 104, 105 and 106 which are relatively movably arranged concentrically with each other and is designed to mold stepped mold products.
As shown in FIG. 27, a mold 100 and the first and second lower punches 104 and 105 are lifted by an air cylinder, at the position where the raw material powder 101 is filled in the mold 100 as shown in FIG. 27A. Next, after a force is applied to the first and second lower punches 104 and 105 by a pressing-down force of the upper punch assembly 102, the first and second lower punches 104 and 105 are moved downward to bring them into abutment against an unillustrated stopper, thus the pressing being completed. A compression force of each one of a plurality of stepped sections 111 ... is adjusted by setting the air pressure of the air cylinder to an appropriate magnitude during the pressing process.
However, although in the above-described prior art, the powder pressing is started by the operation of the upper punch assembly 102 during the pressing process when the state shown in FIG. 27B changes to that shown in FIG. 27C, the density of a first stepped section 111 between the first lower punch 104 and the upper punch assembly 102 increases, and at the same time, powder in the first stepped section 111 falls to a second stepped section 112 between the second lower punch 105 and the upper punch assembly 102. When the pressing force of the upper punch assembly 102 exceeds the filling force of the first lower punch 104, the upper punch assembly 102 and the first lower punch 104 move downward at the same time, as shown in FIG. 27D. Furthermore, as shown in FIG. 27E, the upper punch assembly 102 and the first and second lower punches 104 and 105 are moved downward and brought into abutment against the stopper, thus the pressing being completed. Before the pressing is completed, however, powder in the second stepped section 112 falls to the third stepped section 113 between a third punch 106 and the upper punch assembly 102.
In this way, according to the prior art, the powder solidifies while it is being moved from the first stepped section 111 to the second stepped section 112 and then further to the third stepped section 113 as the pressing proceeds. As a consequence, a crack 114 sometimes occurs in the corner formed between the first stepped section 111 and the second stepped section 112 and in the corner formed between the second stepped section 112 and the third stepped section 113.
In addition, since, the filling depth based on a calculation and, the amount of pressing must be adjusted, much time and high molding accuracy are required to obtain good mold products.